Using photovoltaics to generate and "dump" heat into a distant coop

[sksmass]

I have been thinking about doing a fun chicken-related experiment with solar but am having trouble believing my calculations are right! The idea is to tap solar energy to heat a bucket of water in the coop which would act as a “heat dump” to provide passive winter-time heat to a distant coop.

If I ran two hobby-sized encapsulated photovoltaic panels in series (each 6 volt, 50mA), like these it would generate (assume ideal conditions) 12V of DC current at 50 mA.

Then feed that current directly as a diversion load into a low-voltage water heating element (resistance=0.96 ohm) kind of like this .

So, according to Joules Law (Q=I^2*R*t) this should generate 0.0024 watts=(0.05amps^2*0.96ohms*1 second) per second under ideal conditions. This is ~8.64 watts/hr.

8.64 watts/hr is approximately equal to 29.5 BTU/hr.

Assume 8 hours of sunlight per day and that would be ~236 BTU/day

A BTU is the amount of energy required to raise 1 pound of water 1 degree Fahrenheit.

So, this setup would produce enough passive heat energy to raise a 5 gallon pail of water (which weighs 42lbs) ~5.6 degrees. This heat would be passively lost to the interior of the coop as the water cooled at night. The idea isn't to make the coop toasty, but rather to provide the marginal edge needed to keep chickens a bit more comfortable than they would be in a completely unheated coop.

But I am thinking my calculations must be off somewhere! Any electrical engineers in the house? Tell me where I've gone wrong.

 

[MoSo]

Look into solar thermal instead - sun warms water (or glycol) directly. Thermal panels are better than 75% efficient at capturing solar, whereas PV is less than 20% - and then you take that energy and heat water with it, which has even higher losses. Somewhere up on the Mother Earth News site is a DIY article about making your own solar thermal panels. I plan on making 2 fair-sized ones when I get my greenhouse in. It's pretty much copper tubing coiled up, painted black and put in a box with a glass top - that'll act as a thermosiphon when looped through a small enough water tank

We have 2 Heliodyne 4x8 thermal panels and they work so well we shut off our electric water heater in mid April and don;t turn it back on until late Oct/early Nov. In the summer we have to cover at least one panel because the water temps start getting up above 180 F.



And yes, you have violated Ohm's Law in your calculations:


Power = voltage x current

P = 12V * 50mA = 600mw or .6w aka not enough to light a lightbulb.

Also, the spouse (EE in the family) says that those panels will not run that load - the internal resistance of the panels is too high.

 

[patandchickens]

Yes, if your whole point is to heat some water as thermal ballast for nighttime, there is absolutely no reason to involve electricity at all. Not only is passive solar much simpler and cheaper, it will actually be more efficient per unit area of collector.

Look into the popcan style solar heating panel, or the kind where you run long coils of copper piping with water in it through a flat collector-type space for heating water. Either woudl work; a popcan style heating panel, that just heats AIR, would be simpler and more foolproof (vs freezing!) and you could just route the hot air outflow through or around a buncha water-filled jugs, or something like that.

The popcan style heating thingies really do WORK btw. For wintertime I plastic-wrap my front run, a lean-to shape with 4x7 footprint and about 8' high at the wall that includes both a (low) popdoor and a (high) window. It heats my 15x40' slab-floored building (admittedly, very well insulated) by 5 C or more on a sunny day, even with popdoors open... imagine what it would do in a smaller building, and if you routed that heat through better thermal ballast...

Good luck, ahve fun,

Pat

 

[Nifty-Chicken]

I give this thread a huge:

 

[RocketDad]

Recalculate with 4 to 5 hours of usable sunlight, not 8.

Photovoltaic Pros: no liquids in motion.
Cons: low efficiency per square foot; high cost per watt

Liquid heat collection Pros: High efficiency per square foot.
Cons: system complexity (including the need for electricity for pumping) and risk of leaks or freezing

Air Thermosiphon Pros: Cheap, simple; failure modes non-damaging to structures.
Cons: Thermosiphons can reverse at night.


I did the math once and could heat my entire garage to an uncomfortable level with solar if I covered the entire south wall with a collector. Someday, maybe. Any solar heating experiment should start with heat retention - insulate the heck out of whatever you want warmed. Otherwise the heat you collect will dump back out as soon as the sun goes down.


Do your homework on this, and I don't recommend any of the MotherEarthNews plans or ideas- the ones I've looked at are vague, impractical, idealistic, dated and poorly executed. Even my hippy mom complains that Mother Earth News has become excessively "hippy-dippy."

 

[patandchickens]

Quote:
The thermosiphon will not reverse *much* if you put flaps against a grille, to prevent backwards movement of air; and frankly since normal people will check the coop in the evenings and morning, it is quite easy to just manually open covers to the openings (that is what I do with my 'greenhousified' front run -- I open the popdoor and window when it's up to temp and close it when it gets to be nightfall or leave it closed on a cloudy day. If this were done on a normal popcan-type panel you could use flaps PLUS manual flaps, to give you the best of both worlds.

Any solar heating experiment should start with heat retention - insulate the heck out of whatever you want warmed. Otherwise the heat you collect will dump back out as soon as the sun goes down.

This is a very good point and one I think too often neglected. It only makes sense if you design it as a SYSTEM -- heat collection PLUS lots and lots of insulation PLUS extra heat sink.

Good luck, have fun,

Pat​

 

[Anianna]

Something as simple as painting the roof black in the winter and silver or white in the summer will go a long way toward temperature control.

If you are going to use the solar panels, why not just power a heat lamp with them? Maybe not as fun as your setup, but effective.

 

[patandchickens]

Quote:
what, repaint every 6 months?

Actually, if you are far enough north to have realistic concerns about winter cold, painting the roof black will do near-zero good in terms of wintertime heating, because that roof needs to be well-insulated on the underside. This means that the roof heating up won't transfer much to the interior temperature of the building.

If you are going to use the solar panels, why not just power a heat lamp with them? Maybe not as fun as your setup, but effective.

Sorry, but that will work poorly or not at all.

First, because it takes a LOT of solar panel wattage to run a heat lamp (250 w!), and if you want that lamp running at night you need a good sized bank of expensive large batteries plus a charge controller and so forth. Getting into the thousands of dollars. Cheaper to run electric to the coop

And second, because even if you decide you will buy some large panels but will skip the batteries etc and just have the heat lamps heat water ballast during the day, it is less efficient than a popcan-type panel that heats AIR, because pv panels are relatively inefficient compared to directly heating air, and to a lesser extent because of the losses involved in the electric circuitry.

Honest

Pat​

 

[Anianna]

Quote:
what, repaint every 6 months?

Actually, if you are far enough north to have realistic concerns about winter cold, painting the roof black will do near-zero good in terms of wintertime heating, because that roof needs to be well-insulated on the underside. This means that the roof heating up won't transfer much to the interior temperature of the building.

If you are going to use the solar panels, why not just power a heat lamp with them? Maybe not as fun as your setup, but effective.

Sorry, but that will work poorly or not at all.

First, because it takes a LOT of solar panel wattage to run a heat lamp (250 w!), and if you want that lamp running at night you need a good sized bank of expensive large batteries plus a charge controller and so forth. Getting into the thousands of dollars. Cheaper to run electric to the coop

And second, because even if you decide you will buy some large panels but will skip the batteries etc and just have the heat lamps heat water ballast during the day, it is less efficient than a popcan-type panel that heats AIR, because pv panels are relatively inefficient compared to directly heating air, and to a lesser extent because of the losses involved in the electric circuitry.

Honest

Pat​

I admit that I don't know much about solar panels at this point, beyond doing enough research to determine we couldn't afford to go off grid using solar power. I expect you're right on that, but I have used the paint method effectively in the past, raising the temperature of an 8x10 building 5-10 degrees in cold ranging from mid 20s to about 40 or so degrees. In direct sunlight, it has been very effective, though it does not retain all of that heat at night, the temperature inside was still a few degrees warmer than the outside temp right up until sunup, probably due mostly to body heat. The light paint in the summer was even more effective, cooling the building as much as 20 degrees in direct sunlight. The painting isn't a big deal with a roof that small.

Then again, chickens are pretty cold hardy in general (or so says my research) and it seems to me the real concern is keeping them cool enough when it's really hot.

 

[sksmass]

Thanks for all the good comments and feedback folks. The more I learn about PV the less I think it is a viable solution. And I REALLY don't want to dig up the yard and pay an electrician to run AC current. So I may just go low-tech next winter and merely have a rotating set of black 5 gallon driveway sealer buckets that in the morning I set outside the coop and in the evening put inside the coop. Chickens are pretty tough critters anyway.